Pop pop pop! The sound of bubbles bursting Wednesday morning signaled that a new dark-matter detector is up and running.
The COUPP-60 experiment, based a mile and a half underground at SNOLAB in Canada, searches for dark-matter particles passing through a heated mixture of purified water and CF3I, a chemical used in fire extinguishers. Particles passing through the liquid interact with it, leaving behind a small amount of energy—just enough to cause the liquid to boil.
Dark-matter particles, which scientists think rarely interact with other matter, should form individual bubbles in the COUPP-60 tank, while regular-matter particles such as neutrons interact with atoms in the liquid much more often and should leave a trail of multiple bubbles as they pass through.
Placing the detector so far underground shields it from most of the regular-matter particles that constantly bombard Earth from space. Dark matter, in contrast, is thought to travel through earth and rock undeterred. In addition, researchers submerged the experiment in 7000 gallons of water, which further weeds out neutrons and other “normal” matter.
“The events are so rare, we’re looking for a couple of events per year,” Fermilab postdoc Hugh Lippincott said in a laboratory press release.
Lippincott and his colleagues will continue running the detector and analyze the bubbles to determine if any of them were caused by dark-matter particles.